Regular brewed coffee has at least two important disadvantages compared to instant coffee. First, regular coffee begins to spoil within a short time after brewing while instant coffee does not begin to spoil until, typically, 6 hours after brewing. The slower spoiling rate for instant coffee makes it possible to restore the coffee level in an instant coffee brewer by adding additional instant coffee and a predetermined quantity of water to the remnant of the previous batch with less danger of mixing spoiled and fresh coffee than is present with regular coffee. The slower spoiling rate of instant coffee also permits a large quantity to be brewed and consumed over a long time period without spoiling. This enables larger and fewer batches of instant coffee to be brewed than is possible with regular coffee and reduces waste from spoiled coffee. The less frequent brewing of instant coffee yields a saving in labor costs because less attention need be paid to the coffee brewer. Second, there is a shrinkage of the water volume during the process of brewing regular coffee caused by absorption and retention of water by the coffee grounds. As the amount of water absorbed by the grounds is variable, it is difficult to accurately determine the amount of coffee that will be produced. This may cause either too little or too much coffee to be brewed with the possibilities of users being left without coffee or of the excess coffee being wasted when it spoils.
The above-mentioned disadvantages of regular coffee become especially acute in institutions, e.g. hospitals, where consumption of coffee may be large and spread over extended time periods rather than concentrated in a few short peak periods such as one-hour lunch or dinner servings. Under these circumstances, brewing of large quantities is desirable to minimize labor expenses, and some coffee will stand for a relatively long time period between brewing and consumption. With regular coffee, some coffee may stand for too long and spoil and thus be wasted. The possibilities of wasting coffee are further increased because the inability to accurately predetermine the amount of coffee brewed may inadvertently lead to brewing too much coffee.
These disadvantages of regular brewed coffee can be largely overcome by use of instant coffee for which coffee brewers have special requirements. To most fully utilize the advantages of instant coffee, the coffee brewer should be capable of making large, as well as accurately determinable and easily controllable, quantities of coffee. The instant coffee brewer should also have provisions for easily restoring the coffee level by adding additional instant coffee and more water.
Regular coffee is usually brewed by a so-called "drip" process in which hot water flows by gravity through ground coffee beans. The water extracts the coffee flavor from the beans and leaves behind the coffee grounds. Although mechanical devices, for example, a pump, may be used to raise the water above the coffee beans prior to brewing, the actual brewing process typically requires no mechanical assistance. In contradistinction, instant coffee is typically brewed by placing the instant coffee crystals in the bottom of a cup or pot, adding hot water and stirring until the coffee crystals have dissolved and mixed with the water. If large quantities of instant coffee are to be made, manual or, more likely, mechanical agitation will usually be necessary to dissolve and mix the instant coffee with the water. The mechanical simplicity of the regular coffee maker will thus be lost unless the instant coffee brewer is provided with means for dissolving and mixing the coffee crystals with the water without need for mechanical agitation.